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Linker User Guide
Conventions and feedback
Overview of the linker
Linking models supported by armlink
Image structure and generation
Using linker optimizations
Elimination of common debug sections
Elimination of common groups or sections
Elimination of unused sections
Elimination of unused virtual functions
About linker feedback
Example of using linker feedback
About link-time code generation
Optimization with RW data compression
How the linker chooses a compressor
Overriding the compression algorithm used by the l
How compression is applied
Working with RW data compression
Inlining functions with the linker
Factors that influence function inlining
Handling branches that optimize to a NOP
About reordering of tail calling sections
Restrictions on reordering of tail calling section
About merging comment sections
Getting information about images
Accessing and managing symbols with armlink
Using scatter files
Linker User Guide
About link-time code generation
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| Home > Using linker optimizations > About link-time code generation | |||
Link-time code generation (LTCG) enables
cross source-file optimization by delaying code generation until
the link stage. This can significantly reduce code size. To enable
LTCG, compile your source with -c --ltcg to create
objects in an intermediate format. You must then link these object
files with --ltcg to instruct the linker to perform
code generation.
Both armcc and armlink have
a --ltcg command-line option to enable LTCG.
Note
The LTCG feature is deprecated. As an alternative ARM recommends
you use the --multifile compiler option.
Considerations when using LTCGBe aware of the following when using LTCG:
If no input objects are compiled with
--ltcg, the final object is the same as that produced without LTCG.Debug information is not preserved in any object files compiled with
--ltcg. However, debug information is still preserved for any object files that are not compiled with--ltcg.You cannot use specific object file names in scatter files because LTCG causes a temporary object file to be created and used for linking. If you are using a scatter file, then you must match files using the wildcard
*. Otherwise no match is made on the temporary file.If there is more than one entry point in the input object files provided to the linker, the linker clears all entry points. Therefore, you must specify an entry point on the linker command-line with:
--entry=symbolAny input section containing an entry point is not removed. For example, where an object is built without
--ltcg.Note
The linker option
--entry=object(is not supported when using LTCG.symbol)
The following example shows a typical use of LTCG:
Create ELF object files
one.oandtwo.owith--ltcg:armcc -c --ltcg one.c -o one.oarmcc -c --ltcg two.c -o two.oThe compiler generates intermediate code into the object files.
Link them using the command:
armlink --ltcg one.o two.o -o ltcg_image.axf
The linker:
Combines all the immediate code together, losing the link to original object file names.
Performs code generation for the intermediate code.
Creates the output image.
An intermediate ELF object file generated by link-time
code generation (LTCG) includes a section called .il,
This section is marked with the flags SHF_EXECINSTR and SHF_ALLOC.
Using the intermediate object file one.o from
the example, you can use the fromelf command to view these sections,
for example:
fromelf --text -v one.o
...
====================================
** Section #1
Name : .il
Type : SHT_PROGBITS (0x00000001)
Flags : SHF_ALLOC + SHF_EXECINSTR (0x00000006)
...
Link : SHN_UNDEF
...
====================================
** Section #3
Name : .rel.il
Type : SHT_REL (0x00000009)
Flags : None (0x00000000)
...
Link : Section 2 (.symtab)
Info : Section 1 (.il)
...
====================================
...
- Reference
Linker Reference:
Compiler Reference:
Using the fromelf Image Converter:
| Copyright © 2007-2008, 2011-2012 ARM. All rights reserved. | ARM DUI 0377D |
| Non-Confidential | ID062912 |
 PDF version | |
| Home > Using linker optimizations > About link-time code generation |
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